1. Field of the Invention
The present invention relates to electrophotographic image-forming apparatuses, such as laser beam printers or LED printers.
Due to the recent development in office automation, electrophotographic image-forming apparatuses, such as laser beam printers, have been widely used in computer output terminals, copying machines, facsimile machines or others.
In such image-forming apparatuses, a hard copy is obtained by the steps of charging a photoconductor drum at a predetermined potential by a charger, forming an electrostatic latent image corresponding to an image information on the photoconductor drum by irradiating a light beam thereto, developing the latent image with a toner, transferring the developed image to a recording medium, and fixing the same to the recording medium.
After the transfer step, the photoconductor drum is decharged by a discharger, and the residual toner is scraped off from the surface of the photoconductor drum by a cleaner, thus one cycle of the printing operation is completed on the photoconductor drum.
2. Description of the Related Art
As methods for developing the electrostatic latent image on the photoconductor drum, there are two methods, i.e., two-component developing method using a developer containing carrier and toner and one-component developing method using a developer containing only toner.
One-component developing method is generally simple in construction of the developing apparatus and, therefore, has been widely used for small-sized printers in the recent years. However, for high speed printers, the two-component developing method using a developer containing carrier and toner is still mainly applied.
In a developing apparatus for high speed printers, some problems will occur which might not occur in low or medium speed printers. For example, since a photoconductor drum which holds an electrostatic latent image is rotated in a high speed, the circumferential speed of a developing roller which holds the developer must be increased so as to supply a sufficient amount of developer to the electrostatic latent image so as to properly develop the same.
To attain such a developing apparatus, the developing roller should be rotated in a high speed or the size of the developing magnet roller should be increased to increase the circumferential speed of the developing roller. However, with a single developing roller, it is difficult to supply a sufficient amount of developer to the electrostatic latent image on the photoconductor drum.
As another problem in high speed printers, it is difficult to uniformly supply the developer along the whole axial length of the photoconductor drum to uniformly develop the electrostatic latent image. In particular, it is relatively difficult to supply sufficient toner to the respective end portions of the photoconductor drum.
In an electrophotographic printer, such as LED printers employing magnetic brush developing method, a bias voltage is applied to the developing roller and the developer housing itself is generally charged at the same potential as the bias voltage and, therefore, a toner hopper attached to the developer housing is also in general charged at the same level. Therefore, the toner cartridge cannot be used to supply the toner into the toner hopper during the printing operation.